The need to provide concurrent access to database contents is a recurring requirement in current database technology. Concurrency applies to multiple applications requiring access to the same data at the same time through one database management system (DBMS), and to multiple database systems accessing the same data at the same time. The former circumstance is referred to as the “non-data-sharing environment”, while the latter is denoted as the “data-sharing environment”. Today, databases are increasingly large, and requirements to access their contents are growing geometrically. Further, growing numbers of independent operations are seeking concurrent access.
Typically, database access is provided by way of application programs (“applications”) that employ transactions, basic units of recovery and concurrency in database processing. Concurrent application access is highly desirable.
The virtually universal technique of concurrency control is locking. In this regard, an application will acquire a lock on an object in the database in which it has an interest for reading, inserting, deleting, or changing. In order to ensure that the object will not change while the application is accessing it, the database management system provides a lock giving the application access to the object, while preventing other applications from modifying the object for so long as the application holds the lock.
In a relational database system, contents of a database are represented as tables of data values. Each table corresponds to a relation. In a relational database, a table can be divided into partitions. Each partition contains a portion of the data in the table. A table may be divided into partitions based upon a range of values for a specified key. For example, in the language of the well-known DB2™ database system available from International Business Machines Corporation™, Armonk, N.Y., the syntax of a CREATE TABLESPACE statement includes a NUMPARTS clause that identifies the created table space as partitioned and sets the number of partitions. Partitions on a table in partitioned table space are characterized by a PART clause in a CREATE INDEX statement. Other forms of partitioning a table space are possible. For example, table based partitioning can be used, where a partitioning index is not required.
By partitioning a table, partitions containing more frequently-used data can be placed on faster devices, and parallel processing of data can be improved by spreading partitions over different DASD values, with each I/O stream on a separate channel path. This speeds up and increases the efficiency of access to data in a relational database. However, when access is granted to an application, the entire table is locked, even if only a subset of the data (e.g. data for only a few partitions) will be accessed. When a table is partitioned, locking the entire table may degrade concurrency and database system performance.